Graphite, a carbonaceous material, has a layered structure, and one layer thereof is called graphene. Graphene is comprised of only carbon atoms bonded in the form of a hexagonal lattice. It is only one carbon atom thick and has extremely high thermal conductivity. Graphene holds promise as a two-dimensional nanosheet having strikingly excellent electrical conductivity and other electrical properties.
Nobel Prize recipients Geim et al. repeated the following: sticking graphite flakes to adhesive tape, folding the tape so that the adhesive surface of the tape sandwiches the flakes, and pulling the tape apart to peel off the flakes, thereby yielding graphene (Non-patent Reference 1).
In recent years the following two methods have been used as the main methods of producing graphene from graphite: a method for obtaining graphene oxide by exfoliating in water after oxidizing graphite (Non-patent Reference 2) and a method for obtaining graphene dispersed in a liquid by exfoliating graphite in a solvent or surfactant solution using ultrasound or the like (liquid-phase exfoliation) (Non-Patent References 2 and 3).
In the former (graphene oxide) method, graphite powder must first be oxidized using sulfuric acid, sodium nitrate, potassium permanganate, or the like, and a number of steps are required including the subsequent reduction of the graphene oxide. Those processes, strong acid treatment and subsequent short-time ultrasound application, tend to cause structural defects or fragmentation of graphene, which deteriorate the property of graphene (Patent Reference 1). In contrast, the liquid-phase exfoliation of graphite is thought to be a useful method posing no such problems.
The solvent plays an important role for obtaining the maximum yield in the liquid-phase exfoliation. According to recent studies on graphene exfoliation, the theoretical explanation was provided that a solvent effective for graphite exfoliation has a surface tension of approximate 40 mJ/m2 is theoretically explained (Non-patent Reference 2). However, the level of dispersiveness in one-step exfoliation of graphene using ultrasound is about 2 mg/mL at present and is still inadequate for scaling into a wider range of uses.
Various molten salts have been used as solvents for the liquid-phase exfoliation of graphene because of their high dispersion force and many excellent physical properties. Ionic liquids of imidazolium-based salts in particular have been used suitably as media for disentangling carbon nanotube bundles. The interaction between π electrons of the nanotubes and the organic cation is thought to be important for disentanglement and subsequent gelation. Various ionic liquids have been studied in the liquid-phase exfoliation of graphite from this viewpoint, and the maximum concentration of the graphene dispersion reported to date is 5.33 mg/mL (Non-patent Reference 4).